Researchers found the clinics offer people greater convenience because of their location, efficiency and hours of operation, but are creating a new category of care for people with sudden illnesses, minor injuries or who need things like immunizations.

The study also showed that while the clinics are less expensive than doctor's offices or the ER for minor care, the increasing number of people seeking healthcare as a result of the Affordable Care Act means overall health spending continues to increase.

There are about 2,000 clinics in the United States, receiving more than six million annual patient visits, with locations in pharmacies, grocery stores and big-box stores, as well as stand-alone and in shopping centers, offering minor general care to patients. Because they have weekend and evening hours, and patients can simply walk in without an appointment, they have quickly become popular.

Many insurers encourage customers to use the clinics for minor care, even as they also encourage the establishment of a primary care physician, because previous research has shown they can lower overall population costs for healthcare. In some cases, insurers lower or waive copayments for the clinics to motivate people to use them.

Researchers who participated in the new study contend, however, that any savings have been overshadowed or canceled out by more people seeking new medical care.

"These findings suggest retail clinics do not trim medical spending, but instead may drive it up modestly because they encourage people to use more medical services," Dr. Ateev Mehrotra, an associate professor at the Harvard Medical School, said in a press release. "Retail clinics do offer benefits such as easier access to medical care, but the widely expected cost savings may not be realized."

For the study, published in the journal Health Affairs, researchers analyzed claims and enrollment data for 13.3 million people with Aetna health insurance between 2010 and 2012 in 22 cities with retail health clinics. Aetna covered retail clinic visits during the study period, and researchers reported copayments for clinics were comparable to those for doctor's office visits.

During the two-year period, 519,542 Aetna customers had at least one retail clinic visit. When compared to 861,557 customers who did not use the clinics, the researchers found 58 percent of clinic visits for low-acuity conditions were new uses of care, rather than substitution for more expensive care. The researchers found, however, that clinic users had a reduction in the number of doctor's and emergency room visits they took per year.

While clinic users' costs decreased by $21 per year because of fewer visits to the doctor and ER, going to the clinic increased their costs by $35 -- a net jump of $14 per person per year.

The researchers are careful not to call this bad, comparing more people using cheaper care to the growth of personal computer sales as they got cheaper or more people having certain procedures as laproscopic surgery has become more widespread. More people are seeking care because lower prices mean they can afford it. But with more people in the system, some researchers point out it could appear that care is more expensive.

"The Affordable Care Act is bringing millions of new patients into the system, and it's necessary to provide alternate types of care," Dr. Andrew Sussman, president of the MinuteClinic division at CVS, told the Cincinnati Enquirer.

Calling the study "flawed," he said it "is not an accurate assessment of retail clinic cost savings and value. It is a step backward to think of people who did not have a primary-care physician and get care as excess utilization."

Mehrotra said insurance companies have encouraged the use of clinics as a cheaper alternative to save the industry money, but have not considered that more people means that costs will increase either way. The new utilization, he says, is driving up the overall costs.

"Our findings may impact the decisions of health plans as they decide whether and how to cover care at retail clinics," Mehrotra said. "If the goal is to lower costs, then encouraging use of retail clinics may not be a successful strategy."

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